1. Field of the Invention
The present invention relates to a defect inspection apparatus and, more particularly, to an apparatus suitably used in inspection of defects such as foreign matter attached to the surface of a reticle or a photomask used as an original plate, or the surface of an anti-dust film (pellicle) of the original plate in the manufacture of, e.g., a semiconductor element in a photolithography process.
2. Related Background Art
In the manufacture of a semiconductor element, a liquid crystal display element, or the like in the photolithography process, an exposure apparatus for transferring a pattern formed on a reticle or a photomask (to be referred to as a "reticle" hereinafter) as an original plate onto a photosensitive substrate via a projection optical system is used. When foreign matter larger than a prescribed size becomes attached to the pattern formation surface or to a surface opposing the pattern formation surface of the reticle, or when a pattern on the pattern formation surface includes a defect, a pattern formed on the photosensitive substrate becomes defective. For this reason, before the reticle is attached to the exposure apparatus, the presence/absence of defects including foreign matter, the position of a defect, the size of a defect, and the like must be inspected.
In order to prevent foreign matter from becoming directly attached to the reticle, an anti-dust film called a pellicle is often formed on the two surfaces (or one surface) of the reticle. Even for a reticle formed with such pellicle, the presence/absence of defects including foreign matter, the position of a defect, the size of a defect, and the like on the surface of the pellicle must be inspected.
FIG. 5 shows an example of a conventional defect inspection apparatus. For the sake of explanation, a three-dimensional XYZ coordinate system is also illustrated in FIG. 5.
Referring to FIG. 5, a reticle 1 to be inspected is placed on a table 2 facing its pattern formation surface down. The table 2 is movable in the Y direction by a driving device 3. The moving amount, in the Y direction, of the table 2 is measured by a distance measuring device 4 such as a linear encoder. A light beam L1 emitted from a laser light source 5 is converted into a sheet-like light beam L2 expanded in the X direction via a negative cylindrical lens 6 and a positive cylindrical lens 7. The light beam L2 is radiated onto the surface of the reticle 1, and forms a slit-like illumination region 8 expanding in the X direction on an upper surface 1a of the reticle 1.
If a defect 9 such as foreign matter is present in the illumination region 8 on the upper surface 1a of the reticle 1, scattered light L3 is generated from the defect 9 upon radiation of the light beam L2. The scattered light L3 is focused by a light-receiving lens 10, and an image of the defect 9 is formed on the imaging surface of a one-dimensional image pickup element 11 such as a one-dimensional CCD. The one-dimensional image pickup element 11 has a plurality of light-receiving pixels, and each light-receiving pixel receives light from a predetermined position in the X direction. The coordinate in the X direction (X coordinate value) of the defect 9 on the reticle 1 can be determined on the basis of the position of the light-receiving pixel, which receives the image of the defect, of the one-dimensional image pickup element 11, and the coordinate in the Y direction (Y coordinate value) of the defect 9 can be determined on the basis of the distance measurement output from the distance measuring device 4 at that time. Furthermore, since the one-dimensional image pickup element 11 outputs a pixel output signal having a magnitude proportional to the received light amount, the size of the defect 9 can be roughly determined based on the magnitude of the pixel output signal.
Therefore, the inspection result can be displayed as, e.g., a table which shows the size of the defect in correspondence with the X and Y coordinate values of the defect, or can be displayed as a defect map on the display screen of a CRT display.
However, in the defect inspection apparatus shown in FIG. 5, when a defect on the upper surface (a surface opposing the formation surface of a circuit pattern) 1a of the reticle 1 is to be inspected, the light beam L2 is transmitted through the reticle 1, and is diffracted by a fine circuit pattern on a lower surface (the formation surface of the circuit pattern) 1b of the reticle 1. The diffracted light is focused by the light-receiving lens 10 as if it were scattered light from a defect, and is undesirably detected by the one-dimensional image pickup element 11.
FIG. 6 shows another example of a conventional defect inspection apparatus. For the sake of simplicity, a three-dimensional XYZ coordinate system is also illustrated in FIG. 6.
Referring to FIG. 6, a reticle 1 to be inspected is placed on a table (not shown) facing its pattern formation surface down. A light beam L4 emitted from a laser light source 12 is expanded in one direction by a negative cylindrical lens 13 and a focusing lens 14, thus generating a sheet-like light beam L5. The light beam L5 is obliquely radiated onto an upper surface 1a of the reticle 1 at an angle .alpha., and forms a slit-like illumination region 15 expanding in the X direction on the upper surface 1a of the reticle.
If a defect such as foreign matter is present in the illumination region 15 on the upper surface 1a of the reticle, scattered light is generated from the defect upon radiation of the light beam L5. The scattered light from the defect in the illumination region 15 is detected by a one-dimensional image pickup element 11 via a light-receiving lens 10.
In this case, when the radiation angle .alpha. of the light beam L5 with respect to the upper surface 1a of the reticle is set to be 5.degree. or less, since the transmittance of the reticle 1 becomes considerably small, most of the incident light beam L5 is reflected, and does not reach a lower surface 1b of the reticle, on which a circuit pattern is formed. Therefore, the diffracted light amount from the circuit pattern decreases, and diffracted light from the circuit pattern can be prevented from being erroneously detected as scattered light from a defect on the reticle 1.
In the above-mentioned prior art, the laser light source 5 or 12 is used as a light source. Since the laser light source has a high luminance, even when the slit-like illumination region 8 or 15 is formed, the light amount per unit area (to be referred to as "illuminance" hereinafter) in the illumination region is large, and scattered light from a very small defect can be reliably detected.
However, a light beam emitted from the laser light source is called a Gaussian beam, i.e., the luminance level is highest at the center of the light beam, and is concentrically lowered toward the periphery. For this reason, in both the illuminance distributions of the illumination regions 8 and 15 shown in FIGS. 5 and 6, the luminance level is lowered toward the periphery.
FIG. 7A shows an illuminance distribution S(Y), in the Y direction, of a certain section of the illumination region 8 shown in FIG. 5, and FIG. 7B shows an illuminance distribution S(X), in the X direction, of the section of the illumination region 8 shown in FIG. 5. As shown in FIG. 7B, the illuminance levels at two end portions 8a and 8b in the X direction are considerably lower than that at the central portion. The same applies to the illuminance distribution of the illumination region 15 shown in FIG. 6.
In this case, when the moving speed of the table 2 by the driving device 3 in FIG. 5 is set to always detect scattered light based on a light beam component corresponding to the peak of the illuminance distribution in the Y direction shown in FIG. 7A, the light beam can always be radiated at an almost uniform illuminance level in the Y direction of the reticle 1.
However, in the X direction of the reticle 1, since the luminance level near the center of the reticle 1 is high, and the luminance levels at the two end portions 8a and 8b in the X direction are low, even if the defect size remains the same, the pixel output signal from the one-dimensional image pickup element 11 assumes a different value depending on the attached position, in the X direction, of the defect. Therefore, upon estimation of the defect size from the value of the pixel output signal, a large error may be generated depending on the attached position, in the X direction, of the defect.
In order to eliminate such a drawback, each pixel output signal is multiplied with the reciprocal number of an illuminance corresponding to the position in the X direction as a predetermined correction coefficient in accordance with the detected position, in the X direction, of the scattered light, i.e., the address (a numerical value indicating the position of the pixel) of the light-receiving pixel of the one-dimensional image pickup element 11, thus obtaining an output independently of the position in the X direction.
However, in the defect inspection apparatus shown in FIG. 6, when the height of the reticle 1 changes even slightly, the illuminance distribution with respect to the position in the X direction largely changes. For this reason, even when the above-mentioned correction method is adopted, a satisfactory correction effect cannot often be obtained.
More specifically, in the defect inspection apparatus shown in FIG. 6, since the light beam L5 is obliquely incident on the reticle 1 at the angle .alpha., if the height, in the Z direction, of the reticle 1, changes by .DELTA.h, the illuminance center (the position, in the X direction, on the reticle 1 where the maximum illuminance level is obtained) of the illumination region 15 on the reticle 1 is decentered by (.DELTA.h/tan.alpha.). For example, when .alpha.=5.degree., if the surface of the reticle 1 is shifted by 1 mm in the Z direction, the illuminance center is shifted by 1 mm/tan5.degree., i.e., 11 mm. When the illuminance center of the illumination region 15 changes, the illuminance distribution, in the X direction, of the illumination region 15 also changes. In particular, the illuminance levels near the two ends, in the X direction, of the illumination region 15 change by several fractions to several times upon change in height of the reticle 1.
Therefore, when the height of the reticle 1 changes, and the illuminance center of the illumination region 15 is shifted, even if the pixel output signal from the one-dimensional image pickup element 11 is multiplied with a predetermined correction coefficient, nonuniformity, in the X direction, of defect detection sensitivity still remains.
The above-mentioned drawback is caused by an upward peak pattern (Gaussian distribution) of the illuminance distribution, in the X direction, of the illumination region 8 or 15 on the reticle 1, as shown in FIG. 7B, and can be eliminated by setting the illuminance distribution to have an almost uniform illuminance level independently of the position in the X direction.
As method of obtaining a uniform illuminance distribution, the enlargement magnifications of the lenses 6 and 7 in FIG. 5 and the lenses 13 and 14 in FIG. 6 are increased, so that only the central portion of the Gaussian distribution is used.
However, if a light beam is simply expanded and is radiated onto the reticle 1, the light beam is also radiated onto portions near side surfaces 1c and 1d of the reticle 1 and the table 2, strong scattered light is generated from these radiated portions, and such scattered light cannot be distinguished from that from a defect. Thus, as still another countermeasure, the two end portions of an expanded light beam may be shielded by a light-shielding plate.
FIG. 8 shows main part of an apparatus in which such a countermeasure is taken in the apparatus shown in FIG. 5. Note that a three-dimensional XYZ coordinate system is also illustrated in FIG. 8 for the sake of explanation. Referring to FIG. 8, a light beam L1 emitted from a laser light source 5 and having a Gaussian distribution is expanded in the X direction by a negative cylindrical lens 6A and a positive cylindrical lens 7A, thus forming a light beam L6. The light beam L6 is radiated onto a light-shielding plate 16 formed with a rectangular opening 17, which is elongated in the X direction. A light beam L7 obtained by shielding the two end portions, in the X direction, of the light beam L6 by the light-shielding plate 16 is radiated onto the reticle 1, and forms a slit-like illumination region 18 expanding in the X direction on the reticle 1. Thus, illuminance nonuniformity, in the X direction, in the illumination region 18 on the reticle 1 can be eliminated, a light beam can be prevented from being radiated onto portions near the side surfaces 1c and 1d of the reticle 1, and no unnecessary scattered light is generated.
However, when the light beam L6 is limited by the light-shielding plate 16, as shown in FIG. 8, a diffraction effect when the light beam L6 passes through the rectangular opening 17 poses a problem. Assume that, following the general convention, the width of the Gaussian beam is defined by a width corresponding to a point where the illuminance becomes 13.5% of the maximum illuminance, the width, in the X direction, of the light beam L6 is represented by .DELTA.X, and the width, in the Y direction, of the beam L6 is represented by .DELTA.Y. Furthermore, assuming that the length, in the X direction (the length in the longitudinal direction), of the rectangular opening 17 of the light-shielding plate 16 is represented by a, and the width in the Y direction is represented by b, the following relations are satisfied: EQU .DELTA.X&gt;&gt;a (1) EQU .DELTA.Y&lt;&lt;b (2)
Therefore, no diffraction occurs in the Y direction in the opening 17, and diffraction occurs in only the X direction. For this reason, as is apparent from the diffraction theory, even when the width, in the X direction, of the slit-like illumination region 18 is almost a, the illuminance distribution S(X), in the X direction, in the illuminance region 18 has a fine structure, as shown in FIG. 9.
As shown in FIG. 9, the illuminance distribution S(X), in the X direction, in the illumination region 18 largely changes in a sine-wave pattern at especially two end portions, and has valleys corresponding to extremely low illuminance levels at positions 18a and 18b in the X direction. Therefore, even when the reticle 1 is moved in the Y direction, the illuminance of the light beam is always lowered at the positions 18a and 18b in FIG. 9, and defect detection sensitivity is impaired as compared to that for other regions.